Conventional DC motors typically utilize a multipole permanent magnet stator and a rotor having a plurality of energizable conductors arranged about the surface thereof and parallel to the axis of rotation. Pairs of said conductors are electrically energized by rotating commutator bars provided on the rotor, which commutator bars are electrically energized by a DC source through stationary commutator brushes which make wiping engagement with the commutator bar array. Interaction of the stator permanent magnet fields with the magnetic fields created by DC energization of the rotor conductors provides for rotation of the rotor, whereby relative rotation of the commutator bars and commutator brushes continuously changes the electrical connections between the DC source and the array of conductors in the rotor. Significant wearing of the commutator bars is caused both by the sliding friction of the brushes and the burning action of the commutating current, thereby reducing the useful operating life of the motor.
The above disadvantages, as well as the fact that conventional DC motors have high inertia, has led to the development of the inside out motor design in which the multi-pole rotor is provided with permanent magnet members and the stator is provided with an equal number of poles whose windings are energized by the DC source. This design provides a rotor with lower inertia for a given peak torque, and a stator having greater copper volume and better heat dissipation as compared with conventional DC motor designs. Thus, the inside out motor design has a higher continuous rating in contrast to conventional DC motors of the same size and weight.
The problems of commutation in motors of the inside out type have led to the development of a DC brushless type motor which employs electronic amplifiers and other solid state circuit elements to provide the necessary commutation. The electronic amplifiers and circuit elements required for proper switching of power to the stator windings to generate the rotating field add significant cost and weight to the motor. The solid state switching circuitry also increases motor "cogging" which occurs during low speed motor operation.
The numerous problems and disadvantages encountered in DC motors of both the conventional and inside out design have, in turn, led to the development of the design described in U.S. Pat. No. 3,819,964 which is characterized by providing novel electromechanical switching techniques for commutating the motor windings.
In a preferred embodiment of the invention disclosed in U.S. Pat. No. 3,819,964, the stator assembly is provided with first and second annular conductive rings connected to opposite polarities of a DC source and an annular array of commutator bars disposed proximate thereto. The rotor assembly is provided with a plurality of roller contacts which revolve with the rotation of the rotor shaft to simultaneously couple the opposite terminals of the stator coils to the opposite polarities of the DC source so as to progressively energize the stator coils, the magnetic fields of which interact with the rotating magnetic fields of the rotor permanent magnets to effect rotor rotation.
The commutating technique of the above design exemplified by U.S. Pat. No. 3,819,964 employs a "rigid" roller concept in which the roller contacts are used to bridge between the conductive rings and the commutator bars. These rollers are also mechanically tied together. In this approach, the associated rollers must operate at the same speed. Any factor which results in the production of differing roller or ring diameters or any other condition which would cause one of the rollers to operate at a different speed would impose upon its associated roller the requirement that it must slip with respect to the other. Also, if after long, continued use one of the rollers wears at a rate different from its associated roller causing its diameter to change, slippage will occur. It has also been found that a structure mechanically tying two rollers together introduces dynamic instability in that the moment of inertia of the roller assembly about an axis at right angles to the axis of rotation is quite high and any bounce or eccentricity is greatly magnified at high rotating speeds which can cause the rollers to pull away from the contacting surfaces.
In addition, dust and/or conductive particles developed by wearing of the moving parts and/or introduced from the surrounding environment may settle and collect upon the stator mounted (and hence stationary) commutator array resulting in undesirable and even harmful short-circuiting of adjacent commutator bars.
The rotor permanent magnet assembly also has the disadvantage of being susceptible to demagnetization. Also, for DC machine applications requiring high output shaft velocities it is desirable to operate the roller contacts employed for commutation at lower angular velocities.
The motor, in one preferred embodiment, is of a modular design comprising a hermetically sealed housing containing the stator coils and rotor permanent magnet structure. The commutation structure is coupled thereto and is enclosed in separate accessible housing.